1. Field of the Invention
This invention relates to data communications systems and more particularly to systems for switching data connections between multiple communication ports.
2. Description of the Background Art
Data communication is sometimes facilitated by structuring the communicated data into discrete elements or frames. Each frame contains, in addition to the communicated information, predefined fields for error checking, frame opening and closing, and source and/or destination addressing. Moreover, each frame is typically assembled in accordance with a standard protocol such as HDLC (High-level Data Link Control) for serial communication of frame-level data.
In most frame-based communications systems, frames are communicated between multiple ports. Typically, these frames are communicated between such ports through the use of a single transmission facility such as a multi-drop serial bus which is shared by several receiving ports.
In a multi-drop arrangement, the ports are interconnected with a common bus through which each port shares the same electrical connections with the other interconnected ports. This arrangement permits each port to monitor the bus to determine whether the bus is available for frame transmission. Moreover, this arrangement permits each port to monitor the bus to detect the presence of a frame currently being transmitted on the bus. Each port may also detect where such frame is destined for reception.
The use of a single multi-drop for frame communication between multiple ports is limited, however, to the extent that only two ports may communicate through the common bus at any given time. This limitation frequently results in access to the bus being blocked heavily while a number of ports wait for the bus to become available for frame transmission.
Alternatively to using multi-drop serial buses, other communication systems use conventional parallel interfaces, such as VME (Versa-Module Eurocard), for communication of frames between multiple ports over multi-bit data paths. Such parallel interfaces feature faster frame transmission by providing a dedicated set of electrical connections such as a parallel communication bus between each of the ports over which multiple bits may be transferred simultaneously.
These dedicated sets of electrical connections allow ports to communicate directly with each other without incurring the delays that are necessarily associated with transmitting frames through a common serial bus. While each port must still wait for these dedicated set of electrical connections to become available, frame information is transferred more quickly through the parallel interface once access thereto is obtained.
The use of parallel interfaces for frame communication between multiple ports is limited, however, to the extent that numerous dedicated electrical connections having multi-bit data paths must be installed between each of the ports. Such multi-bit data paths are costlier to manufacture than single-bit data paths which are conventionally used in serial buses.
In addition, a communication system which uses such numerous connections is difficult and costly to make fault-tolerant. This difficulty arises from all ports being vulnerable to a failure on any one of the data paths when a single bit fails during transmission. Typically, fault-tolerance would be achieved by providing redundant data paths for each of the transmitted data and address bits.